Wednesday, July 08, 2020

Brief items - updated

Some further items of note:
  • There is great anxiety and frustration over the latest pronouncement from DHS/ICE about international students in the US.  Let me give a little context.  For many years there has been a rule that international students studying in the US can take no more than 3 credits (or equivalent) per semester of purely online instruction. The point of that was to prevent many people from applying for F visas and then "studying" at online-only diploma mills while actually working. That is, it was originally a policy meant to encourage that student visas go to legitimate international students and scholars pursuing degrees at accredited universities.  In the spring when the pandemic hit and many universities transitioned to online instruction in the middle of the semester, DHS granted a waiver on this requirement.  Well, now they are trying to rescind that, and are doing so in a particularly draconian way: As written, if a university goes online-only, either from the start of the semester or even partway through due to public health concerns, the international students would face having to leave the US on short notice.   This is a terrible, stupid, short-sighted way to handle this situation, and it doesn't remotely serve the best interests of any constituency (student, university, or country).  Unsurprisingly, many many organizations are pushing back against this.  Hopefully there will be changes and/or workarounds.  UPDATE:  The administration appears to have backed down from this.  Hopefully that will stick.
  • On to science.  Quanta has an article about the origins of the rigidity of glass.  The discussion there is about whether there is a kind of hidden structural order in the glassy material.  Fundamentally (as I've written previously), rigidity in any solid results from a combination of very slow timescales for atomic motion (due to lack of thermal energy available to overcome "barriers") and the Pauli principle giving a hard-core repulsion between atoms.  Still, the question of the underlying nature of glassy systems remains fascinating.
  • The 2D materials experts at Columbia have shown clean fractional quantum Hall physics in a monolayer of WSe<sub>2</sub>.  The actual paper is here.  I have yet to come up with a really nice, generally accessible write-up of the FQH effect. The super short version:  Confine charge carriers in strictly two dimensions, and throw in a large magnetic field perpendicular to the plane (such that the energy associated with cyclotron motion dominates the kinetic energy). At certain ratios of magnetic field to number of charge carriers, the charge carriers can condense into new collective states (generally distinguished by topology rather than broken symmetries like the liquid-gas or nonmagnetic/ferromagnetic phase transitions).  The fractional quantum Hall states can have all sorts of unusual properties, but the key point here is that they are fragile.  Too much disorder (like missing atoms or charged impurities), and the energy associated with that disorder can swamp out the energy savings of condensing into such a state.  It's remarkable that the material quality of the monolayer transition metal dichalcogenide (and its encapsulating boron nitride surroundings) is so high.  Seeing how FQH states evolve in this example new material system with rich band structure should be interesting.
  • I feel bad for only now learning about this great series of talks about the state of the art in spintronics, trying to understand, engineer, and control the motion of spin.
  • For your animal video needs, get the behind-the-scenes story about Olive and Mabel here.


Anonymous said...

ICE has really screwed up, but I'm hopeful with the Harvard et al lawsuit that the order may get stopped until after the electio.

Anyways, not really related, but what is the status on the mechanism for superconductivity in twisted bilayer graphene. I was trying to find if someone had done an isotope effect study (all carbon 12 or carbon 13, or one layer with carbon 13) but surprisingly didn't find any study of that sort. Is there some difficulty with an isotope study?

gilroy0 said...

it doesn't remotely serve the best interests of any constituency (student, university, or country)

Ah, Doug, you missed the most vital constituency: the racist xenophobe in the Oval Office and his toady Stephen Miller. It satisfies THEIR needs quite nicely.

Anonymous said...

Thank you very much for the link on spintalks.